Require 'readline' on JRuby

Starting with JRuby 1.7.5, explicit require is required to make use of
jline. (https://github.com/jruby/jruby/issues/1017#issuecomment-24543488)
Merge pull request #94 from BanzaiMan/jruby_require

Require 'readline' on JRuby
Fix "undefined local variable or method `stdscr'" error. This is happening for me in the context of running the knife-essentials chef gem in OSX, which uses highline.
Merge pull request #95 from agentdave/master

Fix "undefined local variable or method `stdscr'" error. This is happeni...
Preparing the 1.6.20 release.